The present invention relates to a method of combustion synthesis, and to intermediate and final products obtained by the method. More particularly, the invention relates to a method of combustion synthesis for exoergic materials which involves use of a plasma spray of exoergic material to coat exoergic, formed green bodies prior to self-propagating high temperature combustion synthesis thereof. It also relates to exoergic green bodies formed during the process and to dense refractory materials formed as a consequence of the process.
Exoergic materials and structures produced therefrom have long been known in the art. Such materials contain all of the components necessary to sustain an exothermic reaction, in and of themselves. Thus, such materials can contain an oxidizing agent and an agent to be oxidized. Such materials can be ignited by suitable means to produce a self-propagating exothermic reaction and have applications as "one-shot" chemical heat sources.
Some exoergic materials, if the amount of exothermic heat released is sufficient, can be ignited in a known combustion regime known as "self-propagating high temperature synthesis" (SHS) or combustion synthesis. Preferably, such a combustion synthesis is carried out under pressure and results in a useful product, preferably a dense refractory material.
The amount of exothermic heat released during combustion synthesis depends upon the particular chemical system utilized. For example, the heat of formation of silicon carbide (SiC) from silicon and carbon powders is 300 cal/g; whereas the heat of formation for titanium diboride (TiB.sub.2) from titanium and boron powders is 1200 cal/g. When the reaction has sufficient chemical energy to be conducted by combustion synthesis, the process is characterized by a rapidly moving combustion front and self-generated high temperatures in the product phase.
The use of a combustion reaction to synthesize a refractory material was first considered by Walton et al. [J. Am. Ceram. Soc., 42(1): 40-49 (1959)] who produced a composite ceramic/metallic material using thermite reactions. In the late 1960's, A. G. Merzhanov and his colleagues began work on self-propagating combustion reactions which led to the development of a process which they called "self-propagating high temperature synthesis" (SHS). [See Merzhanov et al., Dokl. Chem., 204 (2): 429-32 (1972): Crider, Ceram. Eng. Sci. Proc., 3 (9-10): 538-554 (1982).]
Self-propagating high temperature synthesis (SHS), alternatively and more simply termed combustion synthesis, is an efficient and economical process of producing refractory materials. [See for general background on combustion synthesis reactions: Holt, MRS Bulletin, pp. 60-64 (Oct. 1/Nov. 15, 1987); and Munir, Am. Ceram. Bulletin, 67 (2): 342-349 (Feb. 1988).] The combustion reaction is initiated by either heating a small region of the starting materials to ignition temperature whereupon the combustion wave advances throughout the materials, or by bringing the entire compact of starting materials up to the ignition temperature whereupon combustion occurs simultaneously throughout the sample in a thermal explosion.
Advantages of combustion synthesis over prior art methods include: 1) higher purity of products; 2) low energy requirements; and 3) relative simplicity of the process. [Munir, supra at 342.] However, one of the major problems of combustion synthesis is that the products are "generally porous, with a sponge-like appearance." [Yamada et al., Am. Ceram. Soc., 64 (2): 319-321 at 319 (Feb. 1985).] The porosity is caused by three basic factors: 1) the molar volume change inherent in the combustion synthesis reaction; 2) the porosity present in the unreacted sample; and 3) the evolution of adsorbed gases which are present on the reactant powders.
Because of the porosity of the products of combustion synthesis, the majority of the typical materials produced are powders or porous (40-60%) compacts. If dense materials are desired, the powders or compacts then must undergo some type of densification process, such as sintering or hot pressing. The ideal production process for producing dense SHS materials would combine the synthesis and densification steps into a one-step process.
Deposition of powdered materials onto substrates by the use of plasma guns has been known for many years. Exemplary patents are U.S. Pat. Nos. 3,387,110; 3,591,759; 3,676,638; 4,121,083; and 4,146,654. It is to be noted that many of these patents relate to plasma flame-spraying which is entirely nonanalogous to plasma spraying per se.
Although such plasma spraying techniques exist, conventional wisdom in the art has dictated that such techniques cannot be used in the formation of exoergic structures since exoergic materials would be expected to react violently, releasing large quantities of energy, in the plasma. Means of overcoming this problem is described in U.S. Pat. No. 4,806,384. In that patent is described a method of plasma spraying materials capable of self-propagating combustion synthesis onto various substrates.
The plasma spray technique is unsuitable, however, when the substrate itself is comprised of exoergic material because of the presence of absorbed water vapor on surfaces of the powders of the substrate. During subsequent combustion synthesis, the absorbed water vaporizes, creating a porous substrate and fracturing the coating applied by the plasma spray, resulting in defective products.
It would be desirable in the art to provide a method of making plasma sprayed exoergic materials which are dense, substantially non porous and whose exterior coatings are resistant to fracture.